A study on an optical imaging apparatus has been promoted in medical fields. The optical imaging apparatus irradiates a living body with light from a light source such as a laser, and forms an image of information about the inside of the living body obtained from the incident light. The optical imaging technique includes photoacoustic tomography (PAT). The photoacoustic tomography irradiates a living body with pulsed light generated from a light source and detects an acoustic wave generated from a living tissue absorbing the energy of the pulsed light propagating through and diffused in the living body (PTL 1). More specifically, the technique uses a difference in absorptance of light energy between a detected portion such as a tumor and the other tissues, and receives by receiving elements an elastic wave that is generated when part of the analyte absorbs the light energy and instantly expands. By analyzing the detected signal, an optical characteristic distribution, and more particularly, a light-energy absorption density distribution of the living body can be obtained. The information can be used in quantitative measurement for a specific substance in the analyte, for example, glucose or hemoglobin contained in the blood. Hence, the information can be used for specifying the position of a malignant tumor accompanying with the growth of new blood vessels.
The receiving elements of the probe are formed of piezoelectric elements. Variation in reception characteristic may appear among the receiving elements depending on their machining accuracies. Also, variation in reception characteristic may appear among ultrasonic receiving elements using the MEMS technique, which has been developed in recent years, even though the elements are fabricated by a semiconductor process. If the variation in reception characteristic appears among the receiving elements, in particular, if variation in sensitivity characteristic to a frequency band of the photoacoustic wave appears, variation in signal intensity of the photoacoustic wave received by the receiving elements appears among the receiving elements. The photoacoustic apparatus reconfigures an image based on the premise that the receiving elements have the equivalent sensitivities. Thus, if the variation in reception characteristic appears among the receiving elements, the variation in reception characteristic may result in intensity unevenness on a screen after the reconfiguration. The quantitative performance of the measurement may be degraded.
PTL 2 discloses a method for adjusting the sensitivity of an ultrasonic sensor. The ultrasonic sensor transmits an ultrasonic wave with a single frequency from an ultrasonic wave source, which is provided separately from the ultrasonic sensor, to the ultrasonic sensor, and adjusts output signals to decrease the difference (sensitivity difference) between the output signals that are converted by respective elements.